The present invention is drawn to a lithographic mask, particularly a method for forming a lithographic mask used for patterning semiconductor die.
As is known in the art of semiconductor processing, lithographic masks are commonly employed in connection with patterning of resists, such as photoresists, deposited on a semiconductor wafer. By irradiating an energy source through the mask, a pattern formed in a layer on the mask is replicated onto the resist deposited on the semiconductor die or wafer. Energy sources include ultraviolet (UV), deep ultraviolet (DUV), extreme ultraviolet (EUV), as well as x-ray, electron and ion beams. With respect to UV, DUV, and EUV, it is well known that at times the pattern contained within the mask is not exactly replicated on the photoresist of the semiconductor die. For example, diffraction of DUV rays along corners or ends of a feature of the pattern may cause inexact replication of the pattern. Such is illustrated with particularity in connection with the drawings.
Turning to FIG. 1, a basic cross-section of a lithographic mask 1 is illustrated. Lithographic mask 1 includes a substrate layer 10, having a film 12 patterned thereon. The substrate layer 10 may be made of any one of various materials, depending upon the particular energy source employed. For example, in the case of DUV photolithography, film 12 is comprised of chromium, and substrate 10 is formed of fused silica. In this case, the lithographic feature 16 provided within photoresist 14 is transferred to film 12 by etching an exposed region of film 12. Accordingly, upon irradiating the lithographic mask 1 with a DUV energy source, DUV energy passes through substrate 10. Thereafter, a first portion of the energy is absorbed by film 12, and a second portion of the energy is unobstructed by film 12 (i.e., passes through the lithographic feature, corresponding to those portions of film 12 that are patterned). Accordingly, the lithographic feature from the lithographic mask is replicated along the resist provided on the semiconductor die.
It is noted that the general operation of patterning a resist on a semiconductor die utilizing a lithographic mask as described above is set forth in connection with a DUV process. However, the lithographic mask 1 may easily be configured for use with other exposure methods, including EUV, etc. Such masks are well known in the art.
FIG. 2 illustrates an attempt of replicating lithographic feature 16 onto semiconductor die 8, in the case of a negative resist on the semiconductor die 8. Here, it is shown that the lithographic feature 16 is not precisely replicated onto the semiconductor die 8. Indeed, only a portion of the feature is replicated, corresponding to adequately exposed region 18, leaving behind inadequately exposed region 20. This imprecise replication is due to diffraction effects at opposite ends of the lithographic feature known as "line end shortening". In an attempt to address this problem, it is common in the art to incorporate serifs. In this technique, the lithographic feature provided on the lithographic mask 16 is modified to include a serif 26 as illustrated in FIG. 3. By expanding the area of exposure at each end of the lithographic feature, the serif is effective to mitigate the effects of diffraction, and better replicate the desired lithographic feature.
The addition of serifs to the lithographic feature provided on the mask has proved effective. In this regard, generally the database, particularly a computer-aided design (CAD) database, that defines the lithographic feature is modified so as to include serifs. For example, the CAD database may be modified manually by a designer such that the data include serifs. In addition, automated routines are known in the art to add serifs to CAD databases. However, both manual and automated addition of serifs is laborious, as such databases typically contain several gigabytes. As such, addition of serifs in one mask set for a semiconductor die may actually take days to complete, thereby adding cost and complexity to the process for fabricating a mask. The present inventor has recognized a need in the art to provide an improved method of forming a lithographic mask having serifs, which is more cost effective, is relatively simple, and which reduces the cycle time for mask manufacture.